Influence of a Subanesthetic Concentration of Halothane on the Ventilatory Response to Step Changes into and out of Sustained Isocapnic Hypoxia in Healthy Volunteers

Abstract

In humans the ventilatory response to isocapnic hypoxia is biphasic: an initial increase in minute ventilation (VE) from baseline, the acute hypoxic response, is followed after 3-5 min by a slow ventilatory decay, the hypoxic ventilatory decline, and a new steady state, 25-40% greater than baseline VE, is reached in about 15-20 min. The transition from 20 min of isocapnic hypoxia into normoxia results in a rapid decrease in VE, the off-response. In humans, halothane, at subanesthetic concentrations, is known to decrease the acute hypoxic response. In order to investigate the effects of halothane on sustained hypoxia we quantified the effects of 0.15 minimum alveolar concentration halothane on the ventilatory response at the onset of 20 min of hypoxia and at the termination of 20 min of hypoxia by normoxia in healthy volunteers. Step changes in end-tidal oxygen tension were performed against a background of constant mild hypercapnia (end-tidal carbon dioxide tension about 1 mmHg above individual resting values) in fourteen male subjects. The end-tidal oxygen tension was forced as follows: 5-10 min at 110 mmHg, 20 min at 44 mmHg, and 10 min at 110 mmHg. In each subject we performed one trial before and one during 0.15 minimum alveolar concentration halothane administration. Ten responses into hypoxia and nine out of hypoxia were considered for analysis. All control trials were performed during wakefulness. Using behavioral characteristics, the central nervous system arousal state of the subjects during halothane inhalation was defined as "anesthesia-induced hypnosis." The acute hypoxic response averaged 10.4 +/- 4.7 l/min for control versus 3.7 +/- 2.4 l/min for halothane trials (P < 0.01). The hypoxic ventilatory decline was 4.8 +/- 2.5 l/min versus 3.9 +/- 2.9 l/min (NS), the off-response was 6.7 +/- 3.2 l/min versus 3.7 +/- 3.0 l/min (P < 0.05) for control versus halothane, respectively. All values are mean +/- SD. Our results indicate that halothane caused VE to be less than control levels during acute and sustained hypoxia as well as when sustained hypoxia is replaced by normoxia. It is argued that the depression of VE during acute hypoxia is attributed to an effect of halothane on the peripheral chemoreceptors. During sustained hypoxia halothane had no effect on the magnitude of the hypoxic ventilatory decrease, which is probably related to an increase by halothane of inhibitory neuromodulators within the central nervous system. With halothane, the ventilatory decrease when sustained hypoxia is replaced by normoxia is related to the removal of the hypoxic drive at the site of the peripheral chemoreceptors.